Method and apparatus for prevention of slug formation in the casting of film support and sheeting



y 1962 A. E. CZERKAS 3,032,817

METHOD AND APPARATUS FOR PREVENTION OF SLUG FORMATION IN THE CASTING OF FILM SUPPORT AND SHEETING Filed Sept. 17, 1959 Fig-.1

gg' AlfredECzerkas IN V EN TOR.

BYX M ATTORJYEY-S United States The present invention relates to hoppers for distributing cellulose film-forming solutions to form a continuous sheet or film, and more particularly to a new arrangement of the end plates and blades of such hoppers which improves the uniformity of the sheet distributed by the same.

It has been common practice for some time in the art of forming a thin sheeting and film base to spread a thin coating of the desired material, generally a solution of cellulose ester, such as cellulose acetate, in a suitable solvent, on to a heated surface and cause the solvent to evaporate therefrom. The method usually employed consists of feeding the solutions, or dope, from an appropriate feeding device on to the polished metallic surface of a slowly rotating wheel or drum. The solvent evaporates from film more or less progressively as the wheel turns. In less than a complete revolution of the wheel sufficient solvent has been removed from the film to permit it to be stripped from the surface of the wheel and conveyed to a wind-up or carried over the rolls or drums for further curing treatment.

The device usually employed for feeding the solutions, or dope, to the coating wheel comprises a V-shaped hopper provided with spaced-apart blades mounted at an angle to one another forming a discharge slot at the apex of the V. Usually one or both of these blades is adjustable with respect to the other so as to provide a means for controlling the thickness of the dope stream and, therefore, the sheeting formed.

It has been found previously that serious difficulties often arise from the tendency of the cellulose ester solution gradually to form solid slugs, or encrustations, on the edges of the blades, which tend to cause uneven coatiug and to form streaks in the finished product which render it unfit for use. Many elaborate precautions have been undertaken, and arrangements have been provided, to prevent the formation of slugs on the hopper blades and to overcome the difiiculties described above.

The formation of slugs has been especially troublesome at the ends of the hopper blades where the edges of the sheet of wet dope are cast. In this region slugs frequently form and gradually enlarge near the ends of the blades or on the bottom edges of the end plates of the hopper at the blade ends. As these slugs grow larger, they restrict the flow of dope in the ends of the slot between the hopper blades. Eventually, the poor quality of the edges of the cast sheet and concomitant undesirable effects on the over-all coating process necessitate the costly shutdown of the coating machine.

As previously suggested, many different means for preventing slug formation in the discharge slots of such hoppers have been employed. However, they have not been sufiiciently effective or have required costly modifications of the coating hoppers, and complicated controls. The present invention provides a simple, effective means of preventing slug formation.

Therefore, the primary object of the present invention is to improve the performance of the discharge slot of V- shaped hoppers to insure a discharge of a film or dope therefrom which has smooth edges of uniform thickness.

Another object of the invention is to cool the bottom end of the end plates of the hopper and to coolthe ends of the blades of the hopper to prevent the formation of slugs at the ends of the discharge, slot.

A further object is to prevent slug formation at the ends of the discharge slot in the coating of film or sheeting from a solvent-containing dope without undesirably affecting the mechanical, dimensional or chemical properties of the sheet produced.

Another object is to reduce the rate of evaporation of solvent from the edges of the stream of dope cast from the discharge slot of the dope hopper in order to prevent the formation of slugs at these points.

Another object is to condense some of the solvent which evaporates from the edges of the stream of dope cast from the discharge slot of the hopper, and to provide such condensate for lubrication of the dope stream cast from the edges of this slot and for dissolving incipient slugs which may form in or near this slot.

The nOVel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best the understood from the following description of specific embodiments when read in connection with the accompanying drawing in which:

FIG. 1 is an end elevational view of the discharge end of a hopper embodying the present invention and showing it associated with a surface of a wheel onto which the film dope is adapted to be distributed thereby, and

FIG. 2 is a sectional view taken substantially on line 22 of FIG. 1.

Referring to FIG. 1 of the drawing the numeral 10 designates the discharge end of a V-shaped hopper broadly of well known construction adapted to contain and feed a cellulosic solution or dope, which for example, may be a solution of cellulose acetate in a suitable solvent, such as a mixture of methylene chloride and methanol, in the form of a stream S out onto the surface of the casting wheel 11. This feeding device may hereinafter be commonly referred to as the dope hopper and the wheel or drum as the casting wheel, since these terms are customarily employed in the art. The hopper may be adjustably mounted or suspended over the wheel by means, not shown, in such a manner that it may be lowered toward, or raised from, the coating surface as occasion may require.

The discharge end of the hopper comprises a V-shaped trough formed by two inclined blades 12 and 13 closed at opposite ends by end plates 14. The width of the discharge slot 15 formed by the ends of blades 12 and 13 can be adjusted by moving the discharge end 16 of blade 13 toward or away from the discharge end' 17 of blade 12 for regulating the thickness of the stream of dope which flows from the hopper on to the coating surface 11. The blades 12 and 13 are of such width that they abut the inside faces of the end plates 14 of the hopper, see FIG. 2. The ends of the blades andinside surface of the, end plates are machined to form a substantially liquidtight joint. The end plates of the hopper form the ends of .the discharge slot 15 and are contacted by the edges of the dope stream passing through the discharge slot, see FIG. 2.

Hoppers such as that described above are well known in the art and no feature thereof set forth up to this point constitutes anypart of the present inventionexcept insofar as they cooperate with the improvements hereinafter fully set forth and constituting the present invention. In hoppers of the prior art the end plates have been in intimate contact with the metal hopper-blades and the .dope contained in thehopper and are maintained over their entire area at a temperature approximately equal to that of the dope in the hopper.

I have found that the velocity of the emerging stream of dope is lower at the ends of the discharge slot than at other points along the length of the slot. Solvent begins to evaporate from the dope as soon as the dope emerges from the slot. At the ends of the slot, the combination of relatively low dope velocity and relatively rapid solvent loss results in solidification of a minute portion of the dope which acts as a nucleus for the formation of a slug. This incipient slug adheres to the hopper end plate or the edges of the hopper blades and acts as the nucleus for the growth of the slug. Some of the slugs grow large enough to restrict or stop the flow of dope in the ends of the discharge slot of the dope hopper. The characteristics of the edge of the emerging stream of dope are unsatisfactory under these conditions and a costly shutdown of the coating operation is necessary. Precautions heretofore provided for eliminating slugging at the end of hopper discharge slots, such as encasing the area of the hopper blades in a solvent vapor, distributing a thin layer of liquid on the outside edge of the coating slot, and beveling the edges of the discharge ends of the hopper end plates have either been ineffective in preventing slugging or required troublesome controls for supplying solvent at the desired rate to the hopper blades in the liquid or vapor state.

To overcome this deficiency in dope hoppers of the type set forth, and to eliminate a great amount of waste in sheet material caused by this condition, I have provided the simple improvement outlined below. This im provement, while simple in nature, overcomes a deficiency which has been inherent for many years in dope hoppers of the type set forth, and which deficiency has caused a great amount of waste in the sheet material formed by such a hopper. I have found that if the hopper end plates in the area near the ends of the discharge slot in the hopper are cooled to temperatures appreciably below that of the dope in the hopper, the slugging difficulty at the ends of the discharge slot can be eliminated or effectively reduced. The cool portion of the hopper end plate also cools the ends of the hopper blades in the discharge slot by conduction. The edges of the emerging dope stream are formed in the cool ends of the discharge slot. The rate of evaporation of solvent vapors from the emerging dope stream is reduced by lowering the temperature of the dope in this region and the tendency for the formation of incipient slugs is therefore subsequently lessened. In addition, the cool metal surfaces at the ends of the hopper discharge slot condense some of the solvent which does evaporate from the dope stream. This liquid solvent condensate substantially reduces the tendency for the dope to harden at the edges of the blade since this solvent tends to redissolve or keep in solution particles of solidified dope which might otherwise collect on the blades or end plates. The condensed solvent also prevents the accumulation of dope or nuclei of slugs by either insulating them with a liquid solvent or removing them from the proximity of the casting slot. The condensing solvent also tends to lubricate the low velocity dope stream at the interface between the dope stream and the cool metal surfaces of the discharge slot.

For the purpose of illustrating the present invention, I have shown the improvement constituting this invention made in a conventional dope hopper to show the simplicity and ease of incorporating the invention in such a hopper. That area of the lower end of each hopper end plate, shown as 14 in the drawing, which is to be abutted by the discharge ends 17 and 16 of the hopper plates 12 and 13, respectively, is cut off. In place of the cutoff sections there is fitted a water cooled block 18. This block is provided with an internal channel 19 through which cooling water flows from water inlet 20 to water outlet 21. The block is cut out and machined to form a substantially liquid-tight joint with the lower portion 22 of the hopper end plate 14 as well as with the ends of the hopper blades which abut the inside surface of the block. The size of the block 18 will depend upon the amount of adjustment which can be made in the spacing of the blades, because the discharge end of the blades must at all times abut the face of the block.

As shown in the drawing, the bottom of the water cooled block 18 should extend slightly below the discharge ends 16 and 17 of the hopper blades. This prevents the blades from contacting the casting surface 11, clear across the same, if the hopper is accidentally lowered too close to this surface. Also the bottom of the water cooled block should extend below the discharge ends of the hopper blades to prevent the edges of the stream of dope from squirting out sidewise upon leaving the discharge slot, and for the purpose of maintaining a uniform edge on the stream of dope during passage of the same from the discharge ends of the blade to the casting surface.

While I have shown my improvement in dope hoppers applied to hoppers having discharging blades separated from the dope hopper end plates, it will be readily understood that the idea of cooling the metal components at the ends of the discharge slot could be used in a hopper in which the side walls of the hopper itself constituted a discharge slot of a given width.

The operation of this casting apparatus will be further understood from the following description of an example forming a thin sheet from a cellulose ester solution made in a solvent mixture comprising 88 weight percent methylene chloride, 2 weight percent methyl alcohol and 10 weight percent butyl alcohol.

A uniform solution of 100 parts (about 15.2% based on the weight of the entire casting composition) of cellulose acetate of 43.8% acetyl content in 478 parts of methylene chloride, 11 parts of methyl alcohol and 54 parts of butyl alcohol was prepared at room temperature. After the cellulose ester was completely dissolved, there was added 15 parts of triphenyl phosphate as plasticizer. All of the parts specified were by weight.

The resulting solution was filtered to remove the incompletely dissolved particles and fed to the hopper of the film forming apparatus. The discharge end of this hopper 1G is shown in the drawing. The temperature of the dope in the hopper Was maintained at about F. The blades of the hopper were adjusted to feed a stream of Warm dope to the wheel surface of such thickness as to give an eventual film thickness of 0.005 inch. The casting surface was maintained at a temperature of 80 to 90 F. During the flow of the dope through the discharge slot 15 of the hopper, cooling water at an inlet temperature of 60 to 70 F. was fed to the water inlets 20 of the water cooled blocks 18 at each end of the discharge slot. The temperature and rate of flow of the water was adjusted to maintain the temperature of the water cooled blocks at 60 to 70 F.

Dope hoppers of the type described above are sometimes used to cast what is herein termed gel dope onto a cold casting surface. This dope is fluid at the temperatures maintained in the dope hopper. Such dope is capable of setting to a rigid gel at a temperature of approximately 75 F. The temperature of the casting surface is maintained at approximately 65 F., thus permitting the dope to gel immediately upon contacting the casting surface. In casting a gel dope from the dope hopper described above, the temperature and rate of flow of water fed to the water-cooled block 18 are adjusted to cool this block enough to reduce substantially slug formation at the ends of the discharge slot 15 but not enough to cause gelation of the dope in the discharge slot.

Although the improvement constituting the present invention appears to be very simple, and more or less obvious after once being pointed out, the fact that it solves a problem which has confronted those in the film casting art using dope hoppers of the type set forth, for many years, cannot be ignored. The fact that this improvement can be readily adapted to existing dope hoppers with very little trouble and expense widens the range of usefulness of this invention so that it can be applied to any dope hopper of the type set forth which is now in use.

Although I have shown and described certain embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the spirit of the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

l. A hopper for feeding a flowable solvent-containing cellulosic dope to a moving sheet-forming element adjacent thereto, comprising two side plates inclined toward one another; the lower ends of said side plates spaced apart to provide a discharge slot of a given width; two end walls abutting opposite ends of said side plates and extending to the discharge ends thereof to define the length of said discharge slot; and means for cooling the lower ends of said end walls and those portions of the lower ends of the side plates adjacent to said end walls, which the edges of the dope stream leaving the hopper engage, to maintain them at a temperature below the temperature of the solution contained in the hopper and thereby overcome the formation of dope slugs at the ends of the discharge slot.

2. A hopper according to claim 1 in which said last mentioned means comprises providing the lower end of said end walls with a fluid circulating chamber, and means for circulating a cooling fluid through said chamber to maintain the temperature of the surfaces defining the ends of the discharge slot at the desired temperature.

3. A hopper for feeding a flowable solvent-containing cellulosic dope to a moving sheet-forming element adjacent thereto, comprising two side plates inclined toward one another; the lower ends of said side plates spaced apart to provide a discharge slot of a given width; two end walls abutting opposite ends of said side plates and stopping short of the discharge ends of said side plates; a fluid-cooled block fixed to the lower end of each of said end walls and having a face which abuts the lower ends of said side plates and defines the ends of said discharge slot; and means for circulating a cooling fluid through said blocks to maintain the dope-engaging face thereof and the ends of the side plates engaging said face at a temperature below that of the solution contained in the hopper for the purpose of overcoming the formation of dope slugs at the ends of the discharge slot.

4. In the process of making film or sheeting by feeding a solvent-containing fiowable cellulosic dope from a hopper provided with a discharge slot, the step of cooling the edge portions of the stream of dope as it passes through said discharge slot by cooling the surfaces forming the ends of said discharge slot to reduce the evaporation rate of the solvent contained in said edge portions of the dope stream and condense on the surfaces forming the ends of said discharge slot any solvent that does evaporate from said edge portions of the dope stream in order to prevent the formation of slugs of dope at the ends of said discharge slot.

5. In the process of making film or sheeting by feeding a solvent-containing fiowable cellulosic dope from a hopper provided with a discharge slot, the step of cooling the edge portions of the stream of dope as it passes through said discharge slot by cooling the surfaces forming the ends of said discharge slot to reduce the evaporation rate of the solvent contained in said edge portions and thereby reduce the formation of slugs of dope at the ends of said discharge slot.

References Cited in the file of this patent UNITED STATES PATENTS 2,261,486 Phillips Nev. 4, 1941 2,308,034 Van Derhoef 'Jan. 12, 1943 2,369,484 Nadeau Feb. 13, 1945 2,543,287 Hoffstetter Feb. 27, 1951 FOREIGN PATENTS 202,693 Austria Mar. 25, 1959 

